Mosaic versus dual source approaches for modelling the surface energy balance of a semi-arid land

Two-layer parameterisation of the surface energy budget proves to be realis tic for sparse but homogeneously distributed vegetation. For semi-arid land surfaces however, sparse vegetation is usually interspersed by large patch es of unshaded bare soil which may interact directly with the atmosphere wi th little interference with the vegetation. Therefore such surfaces might n ot be realistically represented by a two-layer parameterisation. The object ive of this study is to investigate the issue of representing water and ene rgy transfer processes in arid and semi-arid regions. Two different surface schemes, namely the classic two layer (one-compartment) approach and a two adjacent compartment ('mosaic') approach are used. The performance of both schemes is documented using data sets collected over two sparsely vegetate d surfaces in the San Pedro river basin: homogeneously distributed grasslan d and heterogeneously distributed shrubs. In the latter case the mosaic sch eme seems to be more realistic given the quality of the temperature estimat es. But no clear statement can be made on the efficiency of both schemes fo r the total fluxes. Over each site, we investigate the possibility of artif icially modifying some of the surface parameters in order to get the surfac e fluxes simulated by the one-compartment scheme to reproduce the two-compa rtment ones. The 'cost' associated with this process in terms of surface te mperature estimates is eventually discussed.